In screwing a bolt into a female screw such as a nut, it is desired to accurately match an axis of the bolt with an axis of the female screw and insert the bolt into the screw. In the automobile assembly line and the like, however, in some cases a bolt is forcedly screwed into a female screw by use of a tool such as a power driver in a state where the axis of the bolt is slightly inclined. In such a case, a thread of the bolt fitted into a root part shifted by one pitch from a root part into which the female screw should be fitted, and when the thread is further fitted into the root part in a forced manner, seizure occurs, possibly leading to a problem of not being able to remove the bolt easily.
Since a lot of welding robots are used in the automobile assembly line and the like, a spatter inevitably generated during welding can enter the female screw and engage with the thread of the bolt, crushing the thread. When the bolt is forcedly screwed into the female screw into which the spatter has entered, seizure due to crushing of the thread can occur. Causes for the seizure of the bolt include well-known oblique insertion and invasion of a foreign matter such as the spatter.
Methods for preventing the seizure due to oblique insertion have been devised. According to a typical method, as shown in Patent document 1 of this Applicant, in the case where a guide is formed at a front end of a normal screw portion of the bolt and the bolt is diagonally inserted into the female screw, a front end of the guide is brought into contact with the female screw to correct the position of the bolt. Patent documents 2 and 3 also propose a seizure prevention bolt in which a thread is formed in a guide to improve the position correction effect. However, since the guide has a smaller diameter than the normal screw portion, a screw portion having a smaller diameter than the normal screw portion is inevitably formed at a connection portion between the normal screw portion and the guide, and seizure due to oblique insertion can occur at the screw portion having the smaller diameter.
Any of the conventional seizure prevention bolts described in Patent documents 1, 2, and 3 has no effect of preventing seizure caused by invasion of a foreign matter such as a spatter. For this reason, these bolts are unsatisfactory for screwing in work environments in which the welding spatter disperses, such as the automobile assembly line.